Add piece_extent_affinity to AdvancedSettings

[FranciscoPombal]

And expose option in WebUI settings.

More info:
https://libtorrent.org/single-page-ref.html#piece_extent_affinity

name	type	default
piece_extent_affinity	bool	false

when this is true, create an affinity for downloading 4 MiB extents of adjacent pieces. This is an attempt to achieve better disk I/O throughput by downloading larger extents of bytes, for torrents with small piece sizes

[Chocobo1]

Also you should make sure this PR will build nicely on our CI which is cumbersome work by adding #if conditionals here and there...
As a lazy coder, I would simply put off this PR till the time that specific libtorrent version is available widely.

[FranciscoPombal]

@Chocobo1

Also you should make sure this PR will build nicely on our CI which is cumbersome work by adding #if conditionals here and there...
As a lazy coder, I would simply put off this PR till the time that specific libtorrent version is available widely.

Ugh, I totally forgot this setting is only available in libtorrent >= 1.2.x. So basically I have to wrap everything I wrote in

#if (LIBTORRENT_VERSION_NUM >= 10200)
//code
#endif

right?

[Chocobo1]

right?

You got the idea right but not the version number. Libtorrent seems got that feature in 1.2.2 and you will need to use the correct version format.
Also that leads to another problem: How should WebUI handle this compile time detection?
Or you may just show the option but without any real effects...

[FranciscoPombal]

@Chocobo1

You got the idea right but not the version number. Libtorrent seems got that feature in 1.2.2 and you will need to use the correct version format.

Yeah, right, should be 10202.

Also that leads to another problem: How should WebUI handle this compile time detection?
Or you may just show the option but without any real effects...

I was thinking of changing the description to Use piece extent affinity (requires libtorrent >=1.2.2), so that at least users will get a clue if it does nothing. Otherwise, we could make a request to api/v2/app/buildInfo and conditionally disable stuff based on the libtorrent version, but I am not sure if that is a good solution.

[sledgehammer999]

Being helpful to readers you should have included in the opening comment a link to the docs for piece_extent_affinity and if the docs were small also copy them in a quote.

[FranciscoPombal]

@Chocobo1 Ok now there are #if guards everywhere except:

• in src/gui/advancedsettings.h, where it was not needed and would either require me to put it inline or change the indentation of the file in a weird way
• in the html file, so that checking/unchecking the box in the WebUI does nothing if using libtorrent <1.2.2

[FranciscoPombal]

By the way, I also tested with libtorrent 1.2.1 and verified that it was working as expected.

[Chocobo1]

Only 2 minor comments left

[FranciscoPombal]

@Chocobo1 ready.

[FranciscoPombal]

@glassez Review please?

[FranciscoPombal]

@Chocobo1 I found a better way of addressing your most recent comments.

I moved the libtorrent version checks to the function bodies instead of the whole definitions and declarations, which allowed me to remove the checks in other places.

I did not remove the versions checks from the GUI code because I think it is better that the option does not appear at all if it does nothing. On the WebUI code I had no choice but to always include the checkbox, and as per @glassez's suggestion, I also had no choice on the API side, in order to keep the API interface stable.

[Chocobo1]

@FranciscoPombal
Thank you!

[FranciscoPombal]

@glassez @Piccirello As a result of this PR, the preferences JSON returned by the WebAPI will potentially have one more key value pair, and it is now also be possible to change said field with the api/v2/app/setPreferences method.

I believe this warrants a patch version bump of the WebAPI, since it is not a breaking change for existing clients. It is part of the API contract that the content returned by api/v2/app/preferences is variable.

If it is ok with you, I'd rather this feature also be part of version 2.4.1 of the API, piggy-backing off of #11825